1. Field of the Invention
This invention relates to a motor-drived actuator having an electromagnetic clutch, and in particular to an actuator used for driving a throttle in an automatic cruise control system of automotive vehicles, for example.
2. Description of the Prior Art
Heretofore, there has been used an electromagnetic clutch of a friction type in the motor-driven actuator of this kind. The electromagnetic clutch of this type is so structured as to transmit torque of a motor to an output pulley connected to, for example, a throttle wire by frictional force caused between plane shaped clutch discs. Therefore, there is a problem in that it is not possible to maintain the torque transmittable through the clutch discs constant due to abrasion of surfaces of the clutch discs or slippage between both clutch discs.
In case where teeth or racks are provided on the respective surfaces of the clutch discs so as to be meshed with each other, it is possible to transmit the large torque infallibly. However on the other side, there is the possibility that the throttle valve or the throttle wire may be damaged if the maximum output of the motor is applied directly on them for some reason.